Device and a method for measuring fluid-mechanically effective material parameters of a fluid

ABSTRACT

A method and a device for the measurement of fluid-mechanically effective parameters of a fluid, with a fluid pump which comprises a delivery element ( 2 ) which is mounted in a magnet bearing ( 10, 10   a,    11, 11   a ), according to the invention, envisages the delivery element ( 2 ) of the fluid pump being excited into an oscillation by way of an excitation device ( 16, 44 ), wherein the oscillation parameters as well as, as the case may be, the oscillation behaviour is measured, and parameters of the fluid are determined from this.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 371 nationalization of PCT/EP2010/007442, which inturn claims benefit of U.S. Provisional Application 61/265,007 filedNov. 30, 2009, and European application 09075526.5 filed Nov. 30, 2009.

BACKGROUND

The invention lies in the field of fluid mechanics and in particular offluidic measurement of material characteristics of fluids.

On the one hand, the knowledge of flow resistances in channels throughwhich fluid flows and on the other hand, likewise the exact knowledge ofthe material characteristics of the flowing fluid, thus for example ofthe liquid or of the gas, which is delivered through the channels, areimportant for the assessment and design of fluid-mechanical devices. Therespective characteristics are basically intrinsic of the material, butindividually also dependent on the different constraints such as thetemperature or also the flow speed for example (for example withnon-Newtonian fluids).

It is particularly with applications in medical technology, whenbiologically effective fluids and ones, which are subjected tobiological processes, are to be delivered, that the respectivecharacteristics of these fluids may continuously change. A particularapplication of this type is the delivery of the body's own blood which,with regard to its viscosity, is dependent on different physiologicalprocesses. Accordingly for example, a pump power, with a pump which isapplied for delivering blood, may be adapted to a continuously monitoredblood viscosity.

Different methods for measuring the density and/or the viscosity offluids are known from the state of the art.

Suitable viscometers are for example known from the literature as flowcups, falling body viscometers, measurement agitation drives, and alsofrom the standardisation.

Likewise known are viscosity and density measurement apparatus, whichmeasure the effect of a fluid on an oscillating element which is locatedin this.

Moreover, a measuring method is known from the U.S. Pat. No. 6,581,476B1, with which the rotor of a synchronous motor is driven within a fluidand simultaneously the rotational speed and energy consumption aremeasured for determining the viscosity.

Against the background of the state of the art, it is the object of thepresent invention to provide a method and a device, by way of which onemay measure fluid-mechanical characteristics of a fluid with as littleas possible effort, but in a reliable and accurate manner.

SUMMARY

According to the invention, thereby, a fluid pump is used, whichcomprises a delivery element mounted in a magnet bearing. Moreover, thedevice for measuring the material parameters comprises an excitationdevice for oscillation excitation of the delivery element against acounter-force produced by the magnet bearing, as well as a sensor devicefor measuring the oscillation behaviour of the delivery element.

The device according to the invention therefore, with the application ofa fluid pump which is usually present in any case for the delivery ofthe fluid, permits the measurement of the fluid-mechanically effectivematerial parameters of interest, without a special element having to befitted into the flow path, or samples having to be removed in acomplicated manner. Only an excitation device for the delivery elementand a sensor device for detecting the oscillation behaviour arenecessary. Thereby, a typical delivery element is formed by a rotor,which, depending on the construction type of the pump, delivers fluid inthe axial or radial direction and may be suitably mounted in a magnetbearing in a low-friction manner. Such a magnet bearing may accordinglybe designed as a radial bearing or thrust bearing. An active closed-loopcontrol of the bearing is envisaged, in order to stabilise the positionof the delivery element and to compensate reactions of the fluid to bedelivered on the bearing, on application of the pump power. Such amagnet bearing closed-loop control usually envisages a position sensorfor the delivery element, as well as a control device for controllingadditionally produced magnetic forces, for example by way of anelectromagnet. The control device may be used for example to control thecurrent through a coil producing a magnetic field. The respectiveposition sensor may likewise carry out the measurement of the positionof the delivery element by way of a sensitive magnet coil.

If an active bearing closed-loop control in the direction of the magnetbearing in which the oscillation also takes pace is envisaged, one mustensure that the closed-loop control does not interact with theoscillation in an uncontrolled manner. This for example may be effectedby way of the closed-loop control having a different time constant thanthe oscillation, for example operating at a significantlyhigher-frequency manner or with a much lower frequency than theoscillation.

Also, for the exciting of the delivering element and for sensing itsmovement in response to the excitation, a separate actor and sensordifferent from the sensors and actors of the closed loop control can beused in order to avoid interferences of the viscosity measurement andthe function of the closed-loop control of the magnetic bearing. In oneembodiment, the exciting element independent of the closed loop controlor, in another embodiment, the sensor can be part of the closed loopcontrol of the bearing and the actor can be a separate one. The actorsand sensors for the viscosity measurement, e.g. for the measurement ofthe oscillation properties of the delivering element can act in axial orradial direction of a rotor representing a delivery element or they canrefer to and sense a tilting movement of the rotor.

One may also envisage the bearing closed-loop control interacting withthe oscillation, wherein then the changing bearing forces must be takeninto account with the evaluation of the oscillation.

The device according to the invention for analysis of the oscillation inthe sensor device advantageously comprises a first sensor for measuringan oscillation frequency of the delivery element and/or a second sensorfor measuring the oscillation amplitude of the delivery element as wellas, as the case may be, a time detection device for measuring theoscillation build-up and/or oscillation decay behaviour of the deliveryelement.

Therefore, the measurement of the oscillation properties of thedelivering element can preferably take place in the time domain, wherethe answer of the system is measured with consideration of the point oftime of the beginning or the end of the exciting process. For example,the excitation may be an impulse or a limited number of impulses or asingle rectangular signal and the answer of the system can bethereafter. This has the advantage, that the detected signals are notdisturbed by the exciting signal. Also, it can be an advantage if only afrequency or a damping time and not an absolute value of an amplitude ofan oscillation has to be measured.

The excitation signal may also be periodical, wherein the development ofthe system oscillation from the beginning until reaching a stable statuscan be detected.

The natural frequency after completion of the excitation, as well as theoscillation amplitude or oscillation build-up and/or oscillation decaybehaviour of the free oscillator have a substantial dependency on thedensity and/or the viscosity of the fluid which surrounds the deliveryelement. Moreover, with the measurement of these variables, thegeometric conditions within the fluid pump play a part, if for examplethe fluid is periodically displaced in gaps by way of the oscillation.Thus a calibration is necessary for determining the fluid-mechanicalcharacteristics of the fluid by way of the described measurement.

The physically relevant relationship between the searched,fluid-mechanically effective material parameters (substance values)

ρ density of the fluid in kg/m³

η dynamic viscosity of the fluid in Pas

and the variables which are detectable with regard to measurementtechnology on oscillation of the magnetically mounted rotor after asuitable oscillation excitation, may be described in a simplified in thefollowing way: A movement equation of the form

${{m_{total}*\frac{\mathbb{d}^{2}x}{\mathbb{d}t^{2}}} + {C_{V}*\frac{\mathbb{d}x}{\mathbb{d}t}} + {k*x}} = 0$applies to the rotor oscillating in the direction of the rotation axisof the rotor, the x-axis, with m_(total) the total mass, C_(v) thefriction coefficient, and k the stiffness of the bearing in thex-direction.

It is known that on oscillation of a body in a fluid surrounding it, notonly the body itself, but also a certain share of the adjacent fluidmust be accelerated. The total mass m_(total) thus not only includes theknown motor mass but also an “additional mass”, whose size depends onthe geometry of the arrangement and the density of the surroundingfluid. The size of the “geometry factor” of this “additional mass” isknown for many arrangements.

The force equilibrium which is described in the above oscillationequation is furthermore determined by way of speed-proportional frictionforces. The factor C_(v) is proportional to the searched dynamicviscosity of the fluid.

Accordingly, the searched material characteristics (substance values) ofdensity and dynamic viscosity are hidden in the oscillation equation andmay be determined by way of an analysis of the oscillation behaviour ofthe rotor on the basis of suitable calibrations.

Basically, with the device according to the invention, one may envisagethe excitation device being connected to a device for the closed-loopcontrol of a magnetic bearing force.

With this for example, by way of applying a current to a magnet coil,not only is the magnetic bearing force closed-loop controlled, but alsoa thrust impulse onto a delivery element for exciting an oscillation isgiven.

Also, the first and/or second sensor may be connected to a positionsensor of the magnet mounting of the delivery element, thus for exampleto a sensory magnet coil, constructively unified with this or narrowedor even be identical to it.

With this method, an oscillation is applied onto a delivery element of afluid pump by way of an excitation device, and this delivery element ismounted in a magnet bearing, and the oscillation behaviour of thedelivery element is measured.

Advantageously thereby, the oscillation frequency of the deliveryelement, its amplitude or a decay time of the oscillation or also theenergy expense with the oscillation excitation, are measured.

This may be carried out in a particularly accurate manner with an idledelivery element. However, it may also be advantageous to carry out themeasurement during the fluid delivery, in order to avoid interruptionsof the fluid delivery and despite this, to be able to continuouslymonitor the material characteristics. This is particularly advantageouswith the delivery of biologically effective fluid, in particular inliving bodies, in order not to disturb the respective processes whichare dependent on the supply with the fluid. Finally, the invention alsorelates to the use of a fluid pump for carrying out the measurementmethod of the described type.

The invention is shown and hereinafter described by way of oneembodiment example in a drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

Thereby, there are shown in:

FIG. 1 in a longitudinal section, schematically, an axial pump with aclosed-loop controlled, magnetic thrust bearing,

FIG. 2 schematically, the manner of functioning of the magnetic bearingclosed-loop control and of the pump drive,

FIG. 3 schematically, the measurement of the reaction force on thethrust bearing of the pump in delivery operation, the measurement of therotational speed and, resulting from this with the known fluidcharacteristics, the deduction of fluid-mechanical characteristics of acomponent through which fluid flows and which is different to the pump,as well as

FIG. 4 schematically, the function of the oscillation excitation with anaxial pump, and the measurement of the oscillation and

FIG. 5 a measurement diagram of the oscillation behaviour.

DESCRIPTION OF A PREFERRED EMBODIMENT

FIG. 1 schematically shows a magnet-mounted axial pump in a longitudinalsection, as is applied for example as a blood pump for the human body.The pump is inserted into a cylindrical tube 1 and comprises a rotor 2with rotor blades 3 for advancing a fluid in the flow direction 4.

The drive for the rotor 2 envisages a lamination bundle 5, windings 6,as well as yoke parts 7, 8 which together with the lamination bundle 5,form a highly permeable magnet circuit which is closed via permanentmagnets 9 in the core of the rotor 5, so that as a whole a synchronousmotor or also a brushless d.c. motor with an outer-lying stator isformed, which is electronically commutated.

The rotor 2 is mounted in a magnetically contact-free manner in theaxial direction by way of permanent magnets 10, 10 a whose magnet axisis aligned parallel to the flow direction 4, as well as by way ofstationary permanent magnets 11, 11 a.

The permanent magnets 10, 11 and 10 a, 11 a are in each case alignedsuch that they attract in pairs in the axial direction. With this, atbest a weak equilibrium is formed in the axial direction 4, so thatadditionally, one must provide a closed-loop control device forcontact-free mounting.

For detecting the axial position of the rotor 2, in each case a sensorcoil 14, 14 a is provided in the stationary part of the axial pump 12,13, said stationary part lying in the extension of the rotor 2 withinthe tube 1, opposite which sensor coil, within the rotor, in each case ashort circuit ring 15 a, 15 a lies, so that in each case the distance tothe respective short circuit ring 15, 15 a and thus the axial positionof the rotor 2 may be measured by way of the inductance of the sensorcoils 14, 14 a.

The respective measurement variables are fed as output variables to acontrol device which controls a current through two control coils 16, 16a, in dependence on the measured position or on the difference to thedesired position, of the rotor, said coils strengthening or weakeningthe respective axial fields of the permanent magnets 11, 11 a, in orderto control the attraction force of the permanent magnets 10, 11, 10 a,11 a in a manner such that the rotor 5 assumes a desired position in theaxial direction.

If now the rotor 2, by way of switching on the drive, is set intorotation in a manner such that a fluid located in the tube 1 isdelivered in the direction 4, then a reaction force results, which isopposite to the force acting on the fluid in the direction 4 and whichtemporarily axially deflects the rotor 2.

This procedure is described in more detail by way of FIG. 2, in whichthe elements already shown in FIG. 1, which are equivalent, areindicated with the same reference numerals.

The drive control is indicated in FIG. 2 at 17 and acts on a drivewinding 18 which is shown only very schematically in FIG. 2 and whichsets the rotor 2 into rotation. If it is the case of a synchronous motoras described, then one may already set a rotation speed with the drivecontrol 17. Otherwise, one may additionally provide a rotation speedsensor in the region of the rotor 2, for measuring the rotational speed.

When the rotor 2 is set into rotation, the fluid flows through the tube1 or its continuation, as is represented in FIG. 2 with the arrows 19,20, 21, 22 and 23. The flow resistance parameters of the element 24subjected to throughflow, in the form of an orifice, may be determinedon operation by way of detecting the measurement variables in the regionof the axial pump.

With the operation of the pump drive as is represented above, a force onthe rotor 2 results, which is directed in the direction of the arrow 25and seeks to reduce the axial gap to the stationary component 12. Thesize of this gap is indicated in FIG. 2 with the reference numeral 26,whilst the deviation from the desired position of the rotor, isindicated at 27. This deviation is detected by way of the sensor 28which transmits this variable further to the control/regulation device29 for the axial position of the rotor. This control/regulation deviceaccordingly determines the necessary current i through the control coil16 which influences the axial magnet field acting on at least one of thepermanent magnets in the rotor 2.

In this manner, the axial position of the rotor is controlled with aclosed loop and is held in the middle position in a stable manner.

The current strength i necessary for the control is a measure of thecounter-force acting on the rotor 2, or the pump load, or also thepressure difference in the delivered fluid which is produced by thepump.

The structure of the measurement device and its application formeasuring the flow path is to be represented hereinafter by way of FIG.3.

Thereby, the sensor for the current strength, which is necessary for thethrust bearing stabilisation, is indicated at 36, and 37 indicate thesensors a rotational speed sensor of the rotor 2. The sensors areconnected to the evaluation device 38 which comprises a comparisondevice 39 as well as a memory device 40. The respective characteristicfields for the rotational speed and current and position values arestored in the memory device 40. The evaluation device 38 furthermoreactivates the drive 17 of the rotor. Moreover, the evaluation device 39may obtain information on the type or the viscosity of the appliedfluid, via the input unit 41.

Subsequent to the comparison procedure, the evaluation device 38transfers the evaluated flow resistance parameters to the output device42.

The evaluation, instead of by way of comparison of measured values withthe characteristic field, may also be effected by way of computation byway of an evaluation algorithm.

Hereinafter, it is to be described by way of FIG. 4, how a similardevice may be used within the framework of a measurement of materialcharacteristics. For this, FIG. 4 in a tube 1 schematically and inlongitudinal section, shows a rotor 2 which is axially mounted in amagnetic manner by way of two annular permanent magnets 10, 10 a whichlie in the axial field of a stationary magnet device which is notrepresented in more detail and which produces an axial field, whereinthe controllable magnet coils 16, 16 a serve for axial stabilisation ofthe rotor 2 and may be activated by way of the closed-loop controldevice 29. The coils 16, 16 a thereby produce an additional axial magnetfield for positioning the rotor 2 in the axial direction.

Connected to the control device 29, is an excitation device 43 foroscillation excitation in the axial or radial direction, which foroscillation excitation in the axial direction for example activates oneof the coils 16, 16 a, or for oscillation excitation in the radialdirection, activates an additional excitation coil 44 arranged on theoutside on the tube 1.

An oscillation excitation or a one time excitation in the form of animpulse or a rectangular signal in the axial direction results in anaxial oscillation of the rotor 2 in the axial direction which isindicated by the arrow 45, whilst a radial oscillation or one timeexcitation results in an oscillation in the radial direction indicatedby the arrow 46.

The oscillation behaviour in the case of an axial oscillation on the onehand may be recorded by a sensor coil 47, in whose field region animmersed body 15 of the rotor acts. This sensor coil 47 may beindependent or may also generally serve for bearing closed-loop controladditionally to the position detection of the rotor 2, and in this caseis connected to the closed-loop control device 29, and likewise to themeasurement device 43 for the detection of the oscillation behaviour.

In the case of a radial oscillation which may be initiated by theexcitation coil 44, the radial position of the rotor 2 may be detectedby way of an immersed body 48 and a sensor coil 49 at the outside on thetube 1 and likewise be led to the measurement device 43.

Alternatively to the axial excitation manner represented in FIG. 4, onemay also envisage a separate magnetically effective coil which isdifferent from the coil used for the bearing closed-loop control, beingprovided for applying an excitation signal.

Likewise, the sensor coil which is used for recording the oscillationbehaviour, may be different from the sensor coil for the detection ofthe axial position of the rotor for the bearing closed-loop control.

On mustering a radial oscillation, the problem of the interaction withthe bearing closed-loop control does not occur, as long as the magnetconstellation of the stators located in the tube as well as of the rotor2, is designed in a manner such that the magnet mounting isself-centring in the radial direction. This for example may be the caseby way of the stators in each case carrying circular-disk-like magnetsat their end-sides, which produce an axial magnetic field and which lieopposite the respective magnets 10, 10 a in the rotor 2, which likewiseare aligned axially with their magnetic field and are arrangedconcentrically to these. In this case, the bearing is radiallyself-centring, so that the rotor position, after applying a swingimpulse in the radial direction, is centred on its own again afterrunning through an oscillation with a suitable damping.

The typical oscillation behaviour is represented in the form of theamplitude plotted on the ordinate with respect to time, plotted on theabscissa, is shown in FIG. 5. The rotor assumes the position d₁ betweenthe point in time 0 and t₁, and this position for example represents astable condition between the rotor and a stator.

At the point in time t₁, a periodic swing, for example in the axialdirection, is applied and is maintained as a forced oscillation for atime up to the point in time t₂. A certain amplitude of the oscillationd₃-d₄ results in dependence on the applied power.

The oscillation excitation is switched off at the point in time t₂ andthe oscillation decays approximately exponentially (see logarithmicdecay), whereas practically no or only a defined reduced swing may beascertained at the point in time t₃. The time constant of the decaybehaviour between t₂ and t₃, or for example the time between t₂ and thepoint of thíme, where the amplitude has been reduced to half theamplitude of t₂, may be measured, and describes the dissipation of theoscillation energy which, just as the amplitude, is dependent on theviscosity of the fluid surrounding the rotor. Otherwise, the rotorbetween t₂ and t₃ is located in an oscillation condition of the freeoscillation, in which the frequency of the oscillation or the differenceto the intrinsic frequency without damping or with a specific damping isdependent on the viscosity of the fluid surrounding the rotor andpermits a deduction of the material parameters, for example theviscosity, of the fluid.

The described variables of the oscillation behaviour thus permit theevaluation of fluidic-effective parameters of the fluid, in particularof the liquid which surrounds the rotor.

Thus the invention, with the simplest of means, permits the use of afluid pump which is often present in any case, for the measurement offluidic characteristics of the delivered fluid, and as the case may be,in combination with this, likewise permits information on additionalelements through which the fluid flows, by way of the measurement of thepressure drop in the pump and of the rotational speed.

The invention claimed is:
 1. A device for the measurement offluid-mechanically effective material parameters of a fluid, the devicecomprising: an axial fluid pump which comprises a delivery elementmounted in a magnet bearing, the magnet bearing comprising a permanentmagnet and a first coil; an excitation device for the oscillationexcitation of the delivery element against a counter-force applied bythe magnet bearing, the oscillation excitation in an axial direction inwhich the delivery element is configured to deliver the fluid, theexcitation device comprising a second coil; and a sensor device formeasuring the oscillation behaviour of the delivery element.
 2. A deviceaccording to claim 1, wherein the sensor device comprises a first sensorfor measuring an oscillation frequency of the delivery element.
 3. Adevice according to claim 1 or 2, wherein the sensor device comprises asecond sensor for measuring the oscillation amplitude of the deliveryelement.
 4. A device according to claim 1 or 2, wherein the sensordevice comprises a time detection device for measuring at least one ofan oscillation build-up behaviour or an oscillation decay behaviour ofthe delivery element.
 5. A device according to claim 1 or 2, wherein theexcitation device is connected to a device for the closed-loop controlof a magnetic bearing force.
 6. A device according to claim 2, whereinthe first sensor is connected to a position sensor of a magnet mountingof the delivery element.
 7. A device according to claim 1 or 2, whereinthe delivery element is designed as a rotor.
 8. A device according toclaim 7, wherein the magnet bearing is configured to support thedelivery element in the axial direction.
 9. A device according to claim1 or 2, wherein the magnet bearing is configured to support the deliveryelement in a radial direction perpendicular to the axial direction. 10.A method for measuring one or more fluid-mechanically effective materialparameters of a fluid by way of a fluid pump which comprises a deliveryelement which is mounted in a magnet bearing, wherein the magnet bearingcomprises a permanent magnet and a first coil, the method comprising:exciting an oscillation of the delivery element , by way of anexcitation device, against a counter-force applied by the magnetbearing, a closed-loop control of the magnet bearing having a timeconstant different than the oscillation of the delivery element excitedby the excitation device, the excitation device comprising a secondcoil; and measuring the oscillation behaviour of the delivery element.11. A method according to claim 10, wherein the frequency of theoscillation of the delivery element is measured after the end of theexcitation.
 12. A method according to claim 10 or 11, wherein theamplitude of the oscillation is measured.
 13. A method according toclaim 10, or 11, wherein a decay time of the oscillation after theexcitation is measured.
 14. A method according to claim 10, or 11,wherein the energy expense for the oscillation excitation is measured.15. A method according to claim 10 or 11, wherein the measurement iscarried out with an idle, non-rotating delivery element.
 16. A methodaccording to claim 10 or 11, wherein the measurement is carried outduring the fluid delivery.
 17. A method according to claim 10 or 11,wherein at least one of the rotational speed of the delivery element ora reaction force of the produced fluid pressure on a magnet bearing ofthe delivery element is measured, on operation of the fluid pump. 18.The method of claim 10, wherein the exciting the oscillation of thedelivery element comprises causing axial excitation of the deliveryelement by way of the excitation device, the method further comprisingcausing radial excitation of the delivery element by way of the secondcoil.
 19. A device for the measurement of fluid-mechanically effectivematerial parameters of a fluid, with a fluid pump which comprises adelivery element mounted in a magnet bearing, with an excitation devicefor the oscillation excitation of the delivery element against acounter-force applied by the magnet bearing, and with a sensor devicefor measuring the oscillation behaviour of the delivery element, whereinthe magnet bearing comprises a permanent magnet and a first coil, theexcitation device comprises a second coil, and the sensor devicecomprises at least one of a first sensor for measuring an oscillationfrequency of the delivery element or a time detection device formeasuring at least one of an oscillation build-up behaviour or anoscillation decay behaviour of the delivery element.